Hi Everyone:
I am currently working on adding a
holding tank on my primary
head, which currently discharges straight
overboard. I have spent a lot of time perusing old posts here, and have read Peggy Hall's book. I still have a few remaining questions about the design, and I would like to solicit the collective expertise here.
I plan on using a stock tank, 760mm x 510mm x 310mm deep (it will be oriented flat on its side), and with its longest dimension oriented across the
boat. This is probably the worst orientation for having the contents slosh around, but it is pretty much dictated by the space I have for it. My initial thought was to have it sloping slightly downwards towards the centerline. However, my
research has convinced me I should be putting the vent outlet towards the centerline (to minimize sloshing into the hose), and consequently is seems prudent to mount the tank horizontally, so the contents of a nearly full tank don't flow into the event hose when the
boat is flat. Is my logic sound here?
One aspect of the design I have been unable to find comprehensive
advice on is the location of the tank connections, specifically on the outlet side. I am planning on having separate discharges for the
deck pump-out and the sea discharge
pump. What are the pros/cons of having outlets in the bottom of the tank, vs dip-tubes from the top? Intuitively the bottom outlets are simpler, but carry a greater risk of leakage, and
odor permeation of the hoses due to standing effluent. The dip tubes seem to solve these issues, but is there a down-side other that a bit more expensive? I understand that they should touch the bottom, and be cut at a 25 deg angle. This should allow them to drain the tank as empty as the bottom hose, more or less.
My next question relates to the vent hose. I plan on having the vent outlet at the top of the tank, on the inboard end of the tank to minimize waste sloshing into the hose as mentioned previously (note this placement adds a couple of feet to the hose length). I have read Peggy Hall’s
advice that the vent hose should be horizontal, or close to it. I don’t understand the rationale behind this (unless the tank gasses are heavier than air?). In fact, it seems like a vertical section near the tank would have the benefit of having and splashes easily draining back into the tank. I could have a fairly straight run of 4' or so, at something flatter that 45 degrees, but for a few reasons it would be better to have a slightly longer run with an couple of extra bends in the hose. My question is assuming a 5' to 6' run with a couple of modest bends in it, would I benefit significantly from increasing the size from 1" to 1.5"?
My last question regards the use of a y-valve between the
head and the
holding tank. Since I won’t have huge tank capacity (100 litres/26 US gal, plus half that again for the second head), my thoughts are to cheat a bit when capacity will be tight, by sending liquid-only flushes directly out. I appreciate this isn’t strictly
legal in many cases, but it seems like a reasonable ethical compromise in situations where the tank is likely to fill before reaching a pump-out station. I would want this to share a
single through-hull with the tank discharge, so 2 y-valves would be needed (I would only do this arrangement on 1 of the heads). Is this a common approach, or is the trend solidly towards running everything into the tank?
I welcome your thoughts and advice.
Deryk
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